Lingual Orthodontics vs. Buccal Orthodontics

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INTRODUCTION In every orthodontic force system when the line of force does not pass through the center of resistance (Cr), a moment is created [2]. The magnitude and the direction of that moment are dependent on the direction of the applied force, its magnitude, and its perpendicular distance from the Cr [3] (Fig. 1). Many analytic and experimental studies have been carried out in order to determine the location of the Cr. According to most of these studies, the Cr in a single root tooth is located at 0.24–0.55 of the root length measured from the alveolar crest, or slightly apically to it in teeth with more than a single root [3–10]. The location of the Cr depends on many factors, such as the root shape and length [9], the alveolar bone height [11], and the direction and the magnitude of the applied force. The materials and methods were different in all the above-mentioned studies, and therefore we found it difficult to compare their findings. The location of the Cr in a tooth is not related to the technique used to straighten it. Nevertheless, the relationships between the Cr location and the bracket place- ment (buccal or lingual) directly influence the magnitude and the direction of the moments created by the applied forces (Fig. 2). BONDING As in buccal orthodontics (BO), accurate bracket placement in lingual orthodontics (LO) is of the utmost importance. The bonding process in LO is much more complicated, and is usually done using the indirect technique, for the following reasons: 1. The lingual aspects of the teeth show great variability in tooth size and morphology. In addition, the cingulum, the marginal ridge 1 Lingual orthodontic appliances represent an excellent alternative to labial appliances for esthetically conscious patients. The mechanotherapy involved is altered in some respects, because of the difference in the position of the brackets [1]. The purpose of this article is to describe the biomechanical aspects that one must consider when the traditional labial appliances are replaced by lingual appliances. Lingual orthodontics (LO) versus buccal orthodontics (BO): biomechnical and clinical aspects S. Goren, R. Zoizner, S. Geron, R. Romano Orthodontic Department, Israel Defense Force, Tel-Hashomer, Israel Figure 1. The magnitude and the direction of the moment are dependent on the direction of the applied force, its magnitude, and its perpendicular distance from the Cr.

Transcript of Lingual Orthodontics vs. Buccal Orthodontics

Page 1: Lingual Orthodontics vs. Buccal Orthodontics

INTRODUCTION

In every orthodontic force system when the

line of force does not pass through the center

of resistance (Cr), a moment is created [2].The magnitude and the direction of that

moment are dependent on the direction of

the applied force, its magnitude, and its

perpendicular distance from the Cr [3]

(Fig. 1).

Many analytic and experimental studies

have been carried out in order to determinethe location of the Cr. According to most of

these studies, the Cr in a single root tooth

i s located at 0.24–0.55 of the root length

measured from the alveolar crest, or slightly

apically to it in teeth with more than a single

root [3–10]. The location of the Cr depends

o n many factors, such as the root shape

and length [9], the alveolar bone height [11],and the direction and the magnitude of the

applied force. The materials and methods

were different in all the above-mentioned

studies, and therefore we found it difficult to

compare their findings. The location of the Cr

in a tooth is not related to the technique used

to straighten it. Nevertheless, the relationships

between the Cr location and the bracket place-ment (buccal or lingual) directly influence the

magnitude and the direction of the moments

created by the applied forces (Fig. 2).

BONDING

As in buccal orthodontics (BO), accurateb r a c ket placement in lingual orthodontics

( LO) is of the utmost importance. The bonding

process in LO is much more complicated, and

is usually done using the indirect technique,

for the following reasons:

1. The lingual aspects of the teeth show great

variability in tooth size and morphology. Inaddition, the cingulum, the marginal ridge

1

Lingual orthodontic appliances represent an excellent alternative to labial appliances for

esthetically conscious patients. The mechanotherapy involved is altered in some respects,because of the difference in the position of the brackets [1]. The purpose of this article is to

describe the biomechanical aspects that one must consider when the traditional labial

appliances are replaced by lingual appliances.

Lingual orthodontics (LO) versus buccal

orthodontics (BO): biomechnical and

clinical aspects

S. Goren, R. Zoizner, S. Geron, R. Romano

Orthodontic Department, Israel Defense Force, Tel-Hashomer, Israel

Figure 1. The magnitude and the direction of the

moment are dependent on the direction of the applied

force, its magnitude, and its perpendicular distance

from the Cr.

Page 2: Lingual Orthodontics vs. Buccal Orthodontics

anatomy and the steep curvature of the

lingual surface make direct assessment

o f the correct position of the bracke t simpossible [12] (Fig. 3a,b).

2. Since the correct and esthetic alignment

of the buccal surface of the teeth is our

treatment goal, brackets should be placed

at variable distances from the incisal edge,

due to the different angulations on the

lingual aspects [13].

3. A smaller interbracket distance (IBD) inLO, especially in the anterior region, makes

the use of compensatory bends very

d i f ficult [1, 13–15]. Since indirect bonding

i s more accurate, less wire bending is

required (Fig. 3c).

THE WIRE STIFFNESS

When the brackets are placed on the lingual

s u rface instead of on the labial surf a c e s ,

t h e IBD in the anterior region of the arch is

decreased signific a n t l y. The overall ratio of

anterior lingual distance to labial interbracket

distance was calculated to be 1:1.47. Mandi-

bular ratios are significantly smaller thanm a x i l l a ry ratios [1]. The load/deflection ratio

2 Journal of Lingual Orthodontics, Volume 3 Number 1

is determined by the modulus of elasticity of

the wire, the wire cross-sectional area, and

the wire length. The smaller IBD makes the

same wire stiffer in LO than in BO. According

to Moran [1], the decreased IBD associatedwith LO makes a wire approximately three

times stiffer for first- and second-order bends

than when used with BO, and approximately

1.5 times stiffer for third-order bends.

MOVEMENTS IN THE VERTICAL

DIRECTION: INTRUSION–EXTRUSION

OF A SINGLE ROOT TOOTH

When the intrusive–extrusive line of force

passes buccally or lingually to the Cr, a

moment is created. This moment direction

can be clockwise (CW) or counter- c l o c k w i s e

(CCW), depending on the direction of the

force vector relative to the Cr. The tooth’scrown can therefore move lingually or buc-

cally. The magnitude of the moment in LO is

much smaller than that in BO, due to the

smaller distance between the lingual bracket

and the Cr (Fig. 4). One must estimate, for

each treated case, if the line of force is

passing labially, lingually or right through the

Figure 2. (a) The sagittal distance between the buccal bracket (D1) or the lingual bracket (D2) and the Cr determine

the magnitude of the moments created when force is applied. (b) The vertical distance between the buccal bracket

(D3) or the lingual bracket (D4) and the Cr determine the magnitude of the moments created when force is applied.

(a) (b)

D2

D1

D15Distance of a buccalbracket from the Cr in thesagittal plane

D25Distance of a lingualbracket from the Cr in thesagittal plane

D3=Distance of a buccal bracketfrom the Crin the vertical plane

D4=Distance of a lingual bracketfrom the Crin the vertical plane

D4 D3

Cr

Cr

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S. Goren et al Lingual orthodontics (LO) versus buccal orthodontics (BO) 3

C r, and evaluate whether the moments that

will be created by the intrusion–extrusion

mechanics are desired or should be compen-

sated by changing the bracket torque, angu-

lation and inclination at the laboratory stage,

or by clinical modifications. According to

Geron et al [16], applying an intrusive forcei n BO on a tooth that is initially positioned

between retroclination of 20° and proclination

of 45° will create a lingual root movement

(proclination). In LO, labial root moment

(retroclination) will occur when the tooth is

retroclined more than 220°. The composite

pad on the lingual bracket might change thedistance between the point of force applica-

tion and the Cr and can therefore minimize

the moment magnitude and even reverse the

moment direction (Fig. 5).

MOVEMENTS IN THE SAGITTAL

DIRECTION

In both systems, LO and BO, the direction

o f force applied passes relatively far from

t h e C r, and therefore a moment is created.

The moment tends to move the crown in the

force direction and the apex in the opposite

direction. In this sense, there is no difference

between LO and BO. The sagittal force alsocreates a moment in the buccal–lingual

Figure 3. (a) The lingual aspects of the teeth show great variability in tooth size and morphology. In addition, the

cingulum, the marginal ridge anatomy and the steep curvature of the lingual surface make direct assessment of

the correct position of the brackets impossible. (b) In LO, brackets should be placed at variable distances to avoid

the necessity for first- and second-order bends. (c) A smaller interbracket distance (IBD) in LO, especially in the

anterior region, makes the use of compensatory bends very difficult.

(a) (b)

(c)

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d i r e c t i o n, which tends to rotate the tooth.

I n B O, the vector of force passes buccal to

t h e C r, and in the LO, it passes lingual to it.

Therefore, the directions of these rotationsare opposite. For example, retracting a pre-

molar in BO will tend to rotate the tooth in

a distal–lingual direction. In LO, the same

retraction will create a mesial–lingual rotation

of the tooth (Fig. 6). The clinical implication

i s that the over-corrections applied to the

teeth in the laboratory or clinical stages are

opposite in LO and BO.

MOVEMENTS IN THE TRANSVERSAL

DIRECTION

In both systems, the vectors of force are

passing similarly relative to the Cr, and hence

produce similar moments: the moment tends

to create a movement of the crown in theforce direction and a root movement in the

opposite direction. Clinically, it seems that

expansion is easier in LO than in BO, due to

the elimination of posterior occlusion caused

by the anterior bite plane and the equilibrium

change in LO between the tongue and the

lips.

FRICTION

Many studies have been conducted to evaluate

the principal factors that may influence fric-

tional resistance. The most important factorsare bracket and wire materials [17–19], type

and force of ligation [20–22], the relative

bracket–wire clearance [23], and the archwire

size as related to stiffness [24]. The wire and

b r a c ket materials in LO are the same as in

BO.

The type of ligation is different between

LO and BO. In LO, the double overtie iscommonly used with metal or elastic ligatures

in order to hold the wire in the bracket slot.

There is a lack of information concerning the

normal force created when using this ligation

technique.

As mentioned before, the smaller IBD

makes the same wire stiffer in LO than in BO

[1]. According to some researchers [25,26],stiffer wires reduce the binding and thus

reduce the resistance to friction. On the other

hand, Creekmoore [27] and Articolo and Kusy

[28] suggest that stiffer wires will increase

resistance to sliding in tipped brackets. One

must remember, however, that most of the

Figure 4. The magnitude of the moment in LO is much

smaller than that in BO, due to the smaller distance

between the lingual bracket and the Cr (DL) compared

to the distance between the buccal bracket and the Cr

(DB).

Figure 5. The composite pad on the lingual bracket

might change the distance between the point of force

application and the Cr, and therefore minimize the

moment magnitude and even reverse the moment

direction.

DL

DB

Intrusionforce

MomentDirection

Force Direction

MomentDirection

CompositePad

Force Direction-No Moment

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sliding procedure is done clinically at the

posterior regions, where the difference in IBD

between LO and BO is less .

CLINICAL IMPLICATIONS

CL II/1 malocclusion

The classical manifestations of CL II/1 mal-

occlusion are class II molar relationships,

increased overjet, upper incisors normally in-

clined or proclined, and a constricted maxilla

in relation to the mandible [29].

When CL II/1 malocclusion is treated with

LO, attention must be paid to the contacts

between the lower incisors and the bite-planes of the upper incisor brackets. When

the upper incisors are proclined, the vector of

force could pass labial to the Cr, which would

create a moment that will worsen the pro-

c l i n a t i o n. The anterior-bite plane, created

by the LO brackets, can make the expansion

o f the upper dentition easier, due to the

p o s t e r i o r disarticulation. The vertical openingand the CCW rotation of the mandible (down

a n d back) caused by LO induces a Class II

tendency [28].

CL II/2 malocclusion

The classical manifestations of CL II/2 mal-

occlusion are Class II molar relationships,upper incisor retroclination, and an anterior

deep bite [29].

The upper anterior bite-planes in LO create

intrusive forces on the upper and lower

incisors and simultaneously encourage erup-

tion of the posterior teeth. Because the upper

incisors are retroclined, the vector of theanterior intrusive force can pass palatinally to

the Cr and hence worsen the retroclination.

SUMMARY

LO has some advantages and disadvantagesrelative to BO. Among the advantages is the

improved esthetic appearance, and among

the disadvantages is the difficulty in direct

viewing and access. However, most of the

unique features of LO can be considered either

as advantages or disadvantages, depending

on the malocclusion and according to the

specific goals of treatment. Teeth move underthe force we apply according to biomechanical

principles and the biological environment.

B r a c ket position does not change the need

to understand basic biomechanical rules. As

clinicians, we must analyze in each case the

forces we apply and try to predict the tooth

movement.

REFERENCES

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S. Goren et al Lingual orthodontics (LO) versus buccal orthodontics (BO) 5

Figure 6. (a) Retracting a premolar in BO will tend to rotate the tooth in a distal–lingual direction. (b) In LO, the

same retraction will create a mesial–lingual rotation of the tooth.

(a) (b)

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6 Journal of Lingual Orthodontics, Volume 3 Number 1

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Reprint requests: Dr Rafi Romano, Orthodontic Depart-

ment, Israel Defense Force, 34 Habarzel, Tel-Aviv 69710,

Israel.

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